buildifier check and fix fails silently on linux with --noenable_runfiles

[peakschris]

Without runfiles, buildifier check and fix fails silently. The command has no output or errors. It would be good at least to error in this case, if norunfiles mode will not be supported.

This issue can be covered by the test case in #90 if linux platform is enabled for the norunfiles cases.

To reproduce:

> git clone
> cd buildifier_prebuilt/examples/simple
> BUILD <-- make some edits to BUILD, breaking its formatting
> export BUILDIFIER_DIFF=diff
> bazel run //:buildifier.check --noenable_runfiles

outputs:

INFO: Analyzed target //:buildifier.check (66 packages loaded, 293 targets configured).
INFO: Found 1 target...
Target //:buildifier.check up-to-date:
  bazel-bin/buildifier.check.bash
INFO: Elapsed time: 0.406s, Critical Path: 0.00s
INFO: 5 processes: 5 internal.
INFO: Build completed successfully, 5 total actions
INFO: Running command line: bazel-bin/buildifier.check.bash
>